Plastic molded parts are used in many areas of application. As hollow bodies, they are used, for instance, in automotive engineering as fuel tanks or as containers for other liquids. Due to their relatively simple shaping, low weight and corrosion-resistance, plastic tanks are a preferred means for storing liquids. They are expected to be mechanically stable, to have a low weight and to meet the increasingly stringent requirements relating to efficient packaging in automotive construction.
Normally, the plastic containers are made by means of rotational molding using a rotational mold. In a commonly used production method, a weighed amount of plastic material in the form of powder, pellets or micropellets or the like is placed into a hollow mold whose inner surface will define the outer surface of the plastic container. The mold is then made to rotate around two axes that are usually perpendicular to each other. Heat is applied to the rotational melt mold. The rotational speeds of the rotational melt molds are slow so that centrifugal forces have little effect as compared to the force of gravity. The plastic material begins to melt and to adhere to the inside of the rotational melt mold, thereby giving the plastic container its ultimate shape. This very widespread variant of the rotational molding method calls for processing temperatures above the melting or softening temperature of the plastic material.
Consequently, some materials, especially thermoplastics having high melting or softening temperatures, or else thermosetting plastics, are preferably processed in a likewise known manner in such a way that a chemical precursor of the material intended for the molded part, the so-called plastic precursor, is placed as a melt in liquid form into the rotational melt mold where, under rotation while simultaneously being shaped, the melt chemically reacts, especially polymerizes, to form the final plastic material. This method is advantageously used, for example, for the production of molded parts made of polyamide 6 (PA6), polyamide 12 (PA12) or their copolymers, whereby the corresponding lactams, that is to say, for instance, caprolactam and/or laurolactam, are used as the plastic precursors that are present in solid form at room temperature under normal conditions, but that are processed by means of the rotational molding method in the form of a melt having a very low viscosity (order of magnitude of 10 mPas, that is to say, approximately the same as that of water). The process temperature is preferably kept below the melting temperature of the finished plastic.
The rotational molding method also makes use of the polymerization reactions of dicyclopentadiene (DCPD) to form poly-dicyclopentadiene (PDCPD, e.g. Telene® made by Rimtec Corp.) or of cyclic butylene terephthalate (e.g. CBT® made by Cyclics Corp.) to form polybutyleneterephthalate (PBT). Moreover, it is a known procedure to produce molded parts made of polyurethanes (PU) by means of the rotational molding method by reacting diisocyanates and/or polyisocyanates with diols and/or polyols as the plastic precursors.
All of the cited material systems have in common the fact that the produced molded part is made of a plastic material that is only formed during the shaping in the rotational mold from a plastic precursor that is initially present in liquid form in the rotational mold.
German patent application DE 10 2011 009 748 A1 discloses a method for the production of a two-layer plastic in which a mixture containing a polyamide precursor compound, at least one activator and a catalyst is placed into a preheated mold. Immediately before the mixture is placed into the preheated mold, the components are mixed and placed into the mold as a preparation. After the first mixture has at least partially polymerized, a second mixture is placed into the mold. After a further polymerization reaction to form another polyamide layer, the mold is cooled off so that the plastic product can be demolded.
U.S. Pat. Appln. No. 2009/0266823 A1 describes a method for the production of a plastic bladder made of polyurethane that is suited as a liner for fiber-reinforced pressurized tanks. The bladder is produced by means of the rotational molding method from a reactive mixture that, as the plastic precursor, contains Gyrothane® 900 or 909 (essentially a polyetherpolyol) and Raigidur® FPG (essentially a diisocyanate).
A fundamental problem recognized by the inventor in the production of containers is that the rotational melt mold only defines the outer contour of the molded part, but not the inner shape. It is true that, during the production, a theoretical mean wall thickness of the molded part can be established by suitably adapting the added amount of material to the size of the inner surface of the mold, but it cannot be guaranteed that the container will acquire a uniform wall thickness. The wall thickness is always subject to a certain variation. Particularly in the area of inner radii, that is to say, in the areas where the wall of the rotational mold projects into the interior of the mold, wall thicknesses are obtained that are, at times, actually considerably less than the mean wall thickness. The smaller this inner radius is, the larger the extent of this reduction in the wall thickness. In turn, material always accumulates in the area of the outer radii, that is to say, for instance, the outer edge of a plastic container, as a result of which the wall thickness in such areas is above the mean wall thickness. As the outer radius decreases, the extent of the greater thickness of the wall increases. Whereas outer radii lead only to an increased wall thickness, the stability of thin-walled spots in the area of inner radii can be considerably impaired, which diminishes the strength and the durability of the molded part.
Special challenges arise in conjunction with complex shapes such as, for example, integrally shaped lugs or the like. Especially in constricted spaces, for example, in the area of the outer walls that run in parallel at a small distance from each other, bridge formation can occur during the course of the polymerization, thus promoting void formation between the walls. The envisaged contour feature is then incompletely shaped.
Particularly before the backdrop of increasing requirements relating to packaging in vehicles, however, it is often necessary to ideally utilize a complex and intricate installation space in the vehicle or machine, thus entailing a complex tank design. Therefore, the inventor recognized that it is desirable to be able to systematically influence the material distribution, even in molded parts with complicated shapes. In this context, it is advantageous if the wall thickness can be locally increased in specific places of the finished plastic container. Increasing the shot weight of the material is a remedy with very limited benefits, since the additionally employed material essentially only leads to a further increase in the wall thickness in the area of the outer radii, while the wall thicknesses in the area of the thin spots are only minimally improved. In the final analysis, this measure does nothing but increase the material consumption and the weight of the part, so that, particularly in the case of containers and tanks, the available useful volume is reduced.
It is a known procedure to influence the wall thickness distribution by suitably selecting the speed, the rotational speed ratio, the temperature course in the mold and by employing other measures. U.S. Pat. No. 3,417,097, for example, describes a process in which caprolactam in liquid form is placed into a rotational mold, the caprolactam adheres to the inner contour of the rotational mold while the mold is being rotated, and the melt polymerizes to form a molded part. In order to improve the uniformity of the wall thickness, it is proposed to divide the amount of material over at least two metering procedures and to employ a predetermined temperature profile and rotation profile.
However, since in the rotational molding method like —with blow molding and in contrast to injection molding—only the outer surface of the molded part is in contact with the mold, the results that can be achieved with a given geometry of the molded part are fundamentally limited. These effects are particularly pronounced during the processing of monomers that are placed into the rotational mold in the form of a low-viscosity melt and that are polymerized under rotation. The more the geometry of the molded part deviates from a spherical shape, the broader the variation of the wall thicknesses.
U.S. Pat. No. 6,852,788 B2 describes a composition comprising carrier and binder components as well as a plastic powder to be used in rotational molding with plastic powders or pellets on the basis of a thermoplastic sintering process. This composition is applied as a molding compound into the areas of the rotational mold in which the wall thickness of the molded part is supposed to be increased, that is to say, for example, in the area of ribs and screw domes. As the carrier and binder component, it is proposed to use, among other things, polyethylene having a very low density, petroleum jelly, hydrocarbon wax and hydrocarbon tackifiers. As an alternative, thermoplastics that have a low melt-flow index and that have been adapted to the base polymer of the molded part can be used.
This method, however, cannot be used when a plastic precursor in the form of a melt is placed into the rotational melt mold and the polymerization of the plastic precursor is carried out and activated below the melt temperature of the finished plastic. Since the process temperatures are kept below the melt temperature of the finished polymer, the polymer material in powder form that was added along with the composition would not sinter with itself or with the material newly created by the polymerization. Moreover, the production of the molding compound is quite laborious.